Scavenger's answeranswer is a good idea, but #
might be something that is valid in the code like a pre-processor command in C++
that can be part of the actual code sample.
I suggest to have something that is specified before the code block in a special way. The code block is indented with 4 spaces, so the language specifier could be indented 3 spaces (and only mean something when appearing immediately before a code block). This follows the simplicity in the markup rules that already exists.
VB.NET
Dim myVariable As String 'Here is the first code line
myVariable = "Hello world"